The invention relates to a double-acting hydraulic actuator comprising a cylinder body closed at one end by an end wall and at the other end by a cylinder head, a guide piston slidably mounted in the cylinder body, an actuator rod secured to the guide piston and passing in leaktight manner through the cylinder head, hydraulic means for actuating the actuator rod, and mechanical locking means for locking the actuator rod in a predetermined extended position.
In numerous applications, actuators are used for moving parts or tools, and said parts or tools need to be held in a fixed position throughout the duration of an operation.
This applies in particular when the parts in question are elements of a multi-part mold used for injection-molding pieces out of plastics material. In general, each mold element is fixed to the end of an actuator rod and actuating the actuator rod causes the element to move in translation. To prevent the actuator rod from retracting while the plastics material is being injected under high pressure, the moving element is prevented from moving relative to a fixed structure by means of a pin received in a bore formed in the element and in the structure, the bore extending in a direction perpendicular to the axis of the actuator rod. The pin is also handled by a second double-acting actuator. Thus, in order to position and fix a mold element relative to the structure, two double-acting actuators are used having rods that form a right angle, one of the actuators serving to displace the mold element while the other actuator serves to displace the fixing pin. The stresses due to pressure forces during molding are then taken up by the fixing pin.
That technique thus requires a bore to be made in each mold element and it requires two double-acting actuators to be available for each mold element. Above all, it presents the drawback of requiring considerable time both to put the mold into place and to perform unmolding, since the two actuators must be actuated consecutively. When the mold is being put into place, the actuator rod connected to the element is actuated first, and then once the element is in position, the actuator rod connected to the pin is actuated. For unmolding the injected part, the reverse procedure is performed.
DE 12 57 583 discloses a hydropneumatic actuator in which:
a) the cylinder body presents two coaxial cylindrical walls of different inside diameters and interconnected by a frustoconical annular section, the larger-diameter cylindrical wall being placed beside the cylinder head and the smaller-diameter cylindrical wall serving to guide the guide piston;
b) the actuator rod presents an annular protuberance between the guide piston and the cylinder head, the diameter of the protuberance being not more than the diameter of the smaller-diameter cylindrical wall, said protuberance being disposed in register with the larger-diameter cylindrical wall when the actuator rod is in the predetermined extended position;
c) the guide piston has means putting both sides of said guide piston into fluid communication;
d) an annular piston is placed on the portion of the actuator rod that is situated between the guide piston and the annular protuberance, the annular piston being capable of sliding in leaktight manner against the smaller-diameter cylindrical wall and being capable of sliding in leaktight manner against said portion of the actuator rod under drive of pressure agent between a position in which the actuator rod is released in which it bears against the guide piston, and a locked position in which it bears against the annular protuberance, said annular piston presenting a narrowed end section adjacent to the annular protuberance; and
e) the locking means have locking members and means for urging said locking members to move radially, which locking members, in the released position of the actuator rod, are received axially between a radial face of the protuberance and the annular piston and are housed radially between the portion of the actuator rod and the smaller-diameter cylindrical wall, and in the locked position, are received radially between the larger-diameter cylindrical wall and the narrowed end section of the annular piston and are housed axially between said radial face of the protuberance and the frustoconical section of the cylinder body.
In that document, the narrowed end of the annular piston is frustoconical and it terminates in a plane front face. The means for moving the locking members radially comprise chamfered pins passing axially through the annular protuberance and having chamfers disposed between the locking members to move them apart when the pins come to press against the head of the cylinder in the maximally extended position of the actuator rod. In the locked position, locking members bear against the frustoconical annular section of the cylinder body and against the frustoconical narrowed end of the annular piston. That is why that document provides for a spring between the annular piston and the guide piston.
The spring needs to exert considerable force in order to prevent total or partial unlocking of the actuator while the actuator rod is withstanding forces during an injection-molding operation. In addition, the chamfered pins are of a given length, which prevents subsequent modification of the length of the larger-diameter cylindrical wall in order to adjust the extended position of the actuator rod during injection-molding. In addition, the pressure of the fluid must be sufficient to compensate for the forces delivered by the spring.
An object of the invention is to propose a double-acting actuator which makes it possible to position a mold element in an accurate position and to withstand external compression forces exerted on the actuator rod during a molding operation, e.g. an injection-molding operation.
Another object of the invention is to provide a double-acting actuator in which the piston rod can be held stationary mechanically and with precision in a predetermined extended position.
Starting from the above state of the art, the hydraulic actuator of the present invention is characterized by the fact that the narrowed end section of the annular piston is cylindrical and is terminated by a conical surface for co-operating with a conical surface of the locking members so as to urge them radially outwards when the pressure agent acts on said annular piston in the direction for extending the actuator rod.
Thus, in the locked position, when the actuator rod is withstanding forces tending to cause it to retract, the locking members bear against the frustoconical annular section of the actuator body and exert radial forces only on the cylindrical narrowed section of the annular piston. The prior art spring is thus unnecessary and small pressures suffice for moving the actuator rod. The chamfered pins of the prior art are also eliminated.
Because the locking members in the locked position are spaced apart outwards and co-operate with the frustoconical section of the cylinder body, the bearing surfaces may be large in area, and the compression forces exerted axially on the head of the actuator rod can be taken up by the locking members alone, without any help from the pressure agent.
In order to increase considerably the area of the bearing surfaces, the locking members have conical surfaces facing radially outwards, which surfaces, in the locked position, co-operate with the frustoconical section of the cylinder body.
Preferably, the locking members are constituted by two half-rings.
In order to facilitate maintenance of the actuator in the event of the actuator rod buckling, the actuator rod is made up of two separable cylindrical parts connected together by screw-fastening, one of the parts having the annular protuberance and the portion of the piston rod on which the annular piston is mounted, and the other part sliding in the cylinder head.
Thus, in the event of the actuator rod buckling, the integrity of the internal portion of the actuator is ensured, and it suffices to use a spare part to replace the portion which slides in the cylinder head.
In order to enable the locked position to be adjusted accurately, the cylinder body is constituted by two coaxial portions, of which an outer portion includes the cylinder head, means for fixing to a structure, couplings for coupling to a fluid source, and a cylindrical outer wall having inside tapping, and an inner other portion comprises the end wall of the cylinder body and the body of the cylinder chamber, and presents an outside thread for co-operating with said inside tapping so as to enable the predetermined extended position to be adjusted relative to the structure, internal ducts being provided in said two portions between the couplings and the ends of the cylinder chamber.
This disposition presents an additional advantage. Because the couplings are disposed on the external portion fixed to the structure, the external ducts for hydraulic fluid delivery and exhaust are not subjected to any modification during adjustment. They can be constituted by rigid metal ducts instead of using flexible hoses.